1. Field of the Invention
This invention relates generally to semiconductor circuitry and, more particularly, to semiconductor memory devices.
2. Description of the Related Art
Advances in semiconductor manufacturing technology have provided computer chips with integrated circuits that include many millions of active and passive electronic devices, along with the interconnects to provide the desired circuit connections. As is well-known, most integrated circuits include laterally oriented active and passive electronic devices that are carried on a single major surface of a substrate. Active devices typically include transistors and passive devices typically include resistors, capacitors, and inductors. However, these laterally oriented devices consume significant amounts of chip area.
It is desirable to provide computer chips that can operate faster so that they can process more data in a given amount of time. The speed of operation of a computer chip is typically measured in the number of instructions per second it can perform. Computer chips can be made to process more data in a given amount of time in several ways. In one way, the number of devices included in the computer chip can be increased so that it can operate faster because more information can be processed in a given period of time. For example, if one computer chip operates on 32-bit data, then another computer chip that operates on 64-bit data can process information twice as fast because it can perform more instructions per second. However, the 64-bit computer chip will need more devices since there are more bits to process at a given time.
The number of devices can be increased by making the devices included therein smaller, but this requires advances in lithography and increasingly expensive manufacturing equipment. The number of devices can also be increased by keeping their size the same, but increasing the area of the computer chip. However, the yield of the computer chips fabricated in a run decreases as their area increases, which increases the overall cost.
Computer chips can also be made faster by decreasing the time it takes to perform certain tasks, such as storing and retrieving information to and from memory. The time needed to store and retrieve information to and from memory can be decreased by embedding the memory with the computer chip on the same surface as the other devices. However, there are several problems with doing this. One problem is that the masks used to fabricate the memory devices are not compatible with the masks used to fabricate the other devices on the computer chip. Hence, it is more complex and expensive to fabricate a computer chip with memory embedded in this way. Another problem is that memory devices tend to be large and occupy a significant amount of area. Hence, if most of the area on the computer chip is occupied by memory devices, then there is less area for the other devices. The total area of the computer chip can be increased, but as discussed above, this decreases the yield and increases the cost.
Accordingly, it is highly desirable to provide new structures and methods for fabricating computer chips which operate faster and are cost effective to fabricate.